Abstract
Rapid and incessant propagation of waste has become an issue of concern to stakeholders globally due to the threat they pose to public health and the ecological environment. Wastewater and plastic wastes (PWs) are two major wastes generated on a large scale as upshots of anthropogenic activities. When these wastes are released into the environment, they pose serious challenges, especially in low-income countries (LCs), where waste management (WM) systems are practically insufficient, specifically, in the sub-Saharan Africa (SSA) region. The most alarming part of the improper management of waste and indiscreet disposal is that many people are unaware of the negative impact of their actions on public health and the ecosystem. To manage these wastes sustainably and attain sustainable development goals (SDGs) clean city, climate change, responsible production, and consumption etc. calls for a sustainable waste management system (SWMS) for the overall benefit of the public and the environment. This study, therefore, aims at achieving a circular economy (CE) in the waste management (WM) sectors in SSA where advanced materials are developed from PWs, and the resultant products are deployed for the treatment of WW for a sustainable future. To achieve this, trimetallic catalysts of Fe-Co-Mo supported with CaCO3 and MgO were prepared via a wet impregnation process and applied to the recycled PWs for the fabrication of carbon nanotubes (CNTs) via the thermal-catalytic pyrolysis of PWs in a single-stage chemical vapour deposition (CVD) technique. The developed CNTs were further processed into efficient adsorbents for the removal of tetracycline (TC) contaminants from WW. The synthesized CNTs were characterized using XRD, TEM, EDX, SEM, TGA, BET, FTIR and Raman. Acid purification and functionalization were applied to the CNTs and were deployed for the treatment of TC in the synthetic WW. The removal efficiency of TC from the synthetic WW was determined using UV-vis spectroscopy and fitted into Langmuir, Freundlich and Temkin
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isotherms. The kinetic and thermodynamic parameters of the developed adsorbents were also determined. The effects of CaCO3 and MgO as support materials for Fe-Co-Mo catalyst in the production of CNTs from recycled PP and PE were studied. The results obtained showed the impact of the supports on the yield, quality, and morphology of the synthesized CNTs. The CNTs yield from catalysts supported with CaCO3 and MgO were 35.12% and 37.24 % respectively. The average outer diameters of CNTs produced over the catalyst with CaCO3 and MgO supports were 36 and 29 nm respectively. The ID/IG ratio for CaCO3 and MgO supports were 0.6724 and 0.8401 respectively. The change in entropy for all phases were -274.41 and -1032.97 J/mol/K and -213.0, -318.48 and -1022.13 J/mol/K for CaCO3 and MgO supported catalyst respectively which confirmed non-spontaneous endothermic reaction. The comparative studies on the use of recycled PP and PE as feedstocks for CNTs synthesis showed that CNTs obtained have high graphitic values with d-spacing of 0.3425 and 0.3442 nm respectively. The ID/IG ratio for CNTs produced from recycled PP and PE were 0.6724 and 0.9028 respectively. This implied that the CNTs produced from recycled PP as feedstock have higher ordered graphite than those produced from PE. The TGA showed percentage purity of 94.71% and 94.40% for the CNTs obtained from recycled PP and PE respectively. The results of TC adsorption on the developed adsorbent depend significantly on pH and temperature. pH of 4 and temperature of 30 °C were the best adsorption conditions. The Intraparticle diffusion model best fits the kinetic of TC adsorption on the functionalized CNTs (F-CNTs) with an R2 value of 0.9776. TC adsorption on F-CNTs was endothermic and spontaneous with ~94% adsorption efficiency. It was concluded that the use of recycled PP and PE in the fabrication of CNTs and development of adsorbents for the removal of emerging contaminants from WW utilizing CNTs is a promising, effective, and economical route for a SWMS or for CE to be attained in the WM sectors.